Numerical Simulations of Two-Layer Flow past Topography. Part II: Lee Vortices

This study considers a two-layer fluid with constant density in each layer connected by a layer of continuously varying density for flows past topography in which hydraulic jumps with lee vortices are expected based on shallow-water theory. Numerical integrations of the Navier–Stokes equations at a Reynolds number high enough for a direct numerical simulation of turbulent flow allow an examination of the internal mechanics of the turbulent leeside hydraulic jump and how this mechanics is related to lee vortices. Analysis of the statistically steady state shows that the original source of lee-vortex vertical vorticity is through the leeside descent of baroclinically produced spanwise vorticity associated with the hydraulic jump. This spanwise vorticity is tilted to the vertical at the spanwise extremities of the leeside hydraulic jump. Turbulent energy dissipation in flow through the hydraulic jump allows this leeside vertical vorticity to diffuse and extend downstream. The present simulations also suggest a geometrical interpretation of lee-vortex potential-vorticity creation, a concept central to interpretations of lee vortices based on the shallow-water equations.

Mesoscale Vortex Development during Extreme Precipitation: Colorado, September 2013

On 11–12 September 2013, portions of northern Colorado experienced flash flooding as a result of high rain rates accumulating over 180 mm of rain in 6 h. From 0400 to 0700 UTC 12 September a mesovortex was observed traveling northwestward toward the city of Boulder, Colorado, with enhanced upslope flow on its north side and localized deep convection. Although the mesovortex was observed in an area common for lee vortex formation, namely that associated with the Denver Cyclone, it is shown via ARW model simulations that the mesovortex intensified through the release of latent heat, similar to the processes leading to mesoscale convective vortices, rather than by dry topographic-flow dynamics. High rates of cloud water condensation at relatively low altitudes led to a strong vertical gradient in latent heating, resulting in a near-surface positive potential vorticity anomaly. Reducing the contribution of cloud water condensation to latent heating by 50% resulted in no mesovortex development in the model and a substantial decrease in precipitation. On the other hand, removing the topographical forcing in the model did not inhibit the mesovortex formation, confirming the secondary role of topography. The mesovortex enhanced upslope winds and convection, and was thus a key feature in the generation of intense precipitation over Boulder. The ability to forecast the development of these mesovortices and their subsequent environmental and hydrological effects could be critical for decision-makers and the public, given their association with high rainfall rates.

Mechanisms of Banner Cloud Formation

Banner clouds are clouds in the lee of steep mountains or sharp ridges. Their formation has previously been hypothesized as due to three different mechanisms: (i) vertical uplift in a lee vortex (which has a horizontal axis), (ii) adiabatic expansion along quasi-horizontal trajectories (the so-called Bernoulli effect), and (iii) a mixing cloud (i.e., condensation through mixing of two unsaturated air masses). In the present work, these hypotheses are tested and quantitatively evaluated against each other by means of large-eddy simulation. The model setup is chosen such as to represent idealized but prototypical conditions for banner cloud formation. In this setup the lee-vortex mechanism is clearly the dominant mechanism for banner cloud formation. An essential aspect is the pronounced windward–leeward asymmetry in the Lagrangian vertical uplift with a plume of large positive values in the immediate lee of the mountain; this allows the region in the lee to tap moister air from closer to the surface. By comparison, the horizontal pressure perturbation is more than two orders of magnitude smaller than the pressure drop along a trajectory in the rising branch of the lee vortex; the “Bernoulli mechanism” is, therefore, very unlikely to be a primary mechanism. Banner clouds are unlikely to be “mixing clouds” in the strict sense of their definition. However, turbulent mixing may lead to small but nonnegligible moistening of parcels along time-mean trajectories; although not of primary importance, the latter may be considered as a modifying factor to existing banner clouds.

Numerical Analysis of a Mediterranean “Hurricane” over Southeastern Italy

The presence of a subsynoptic-scale vortex over the Mediterranean Sea in southeastern Italy on 26 September 2006 has been recently documented by the authors. The transit of the cyclone over land allowed an accurate diagnosis of the structure of the vortex, based on radar and surface station data, showing that the cyclone had features similar to those observed in tropical cyclones. To investigate the cyclone in greater depth, numerical simulations have been performed using the Weather Research and Forecasting (WRF) model, set up with two domains, in a two-way-nested configuration. Model simulations are able to properly capture the timing and intensity of the small-scale cyclone. Moreover, the present simulated cyclone agrees with the observational analysis of this case, identifying in this small-scale depression the typical characteristics of a Mediterranean tropical-like cyclone. An analysis of the mechanisms responsible for the genesis, development, and maintenance of the cyclone has also been performed. Sensitivity experiments show that cyclogenesis on the lee side of the Atlas Mountains is responsible for the generation of the cyclone. Surface sensible and latent heat fluxes become important during the subsequent phase of development in which the lee-vortex shallow depression evolved as it moved toward the south of Sicily. During this phase, the latent heating, associated with convective motions triggered by a cold front entering the central Mediterranean area, was important for the intensification and contraction of the horizontal scale of the vortex. The small-scale cyclone subsequently deepened as it moved over the Ionian Sea and then maintained its intensity during its later transit over the Adriatic Sea; in this later stage, latent heat release continued to play a major role in amplifying and maintaining the vortex, while the importance of the surface fluxes diminished.

Dynamics of Track Deflection Associated with the Passage of Tropical Cyclones over a Mesoscale Mountain

In this study, the fifth-generation Pennsylvania State University–National Center for Atmospheric Research (PSU–NCAR) Mesoscale Model (MM5) was used to simulate Supertyphoon Bilis (in 2000) and Typhoon Toraji (in 2001) in order to investigate the dynamics of track deflection caused by the Central Mountain Range (CMR) of Taiwan. The MM5 predicted the track of each storm reasonably well. Bilis was stronger and had a relatively faster forward motion, which helped make the track continuous as it crossed the CMR. The use of a “bogus” vortex in the initialization process helped produce a storm closer to the observed strength. Bilis is a classic example of a typhoon crossing Taiwan with a continuous track. For comparison, Typhoon Toraji, a typical typhoon having a discontinuous track, was also studied. Toraji was weaker and had a relatively slower forward speed, which prevented the original low center from crossing over the CMR and forced more air parcels to go around the northern tip of the CMR. As a result, it produced a vortex and a secondary low center on the lee. Potential vorticity banners on the north side of the CMR acted to organize the secondary low and the lee vortex. With time, the low-level circulation extended into the upper levels, completing the formation of the secondary center. Remnants of the initial center crossed over the CMR and were entrained into the secondary center. Nondimensional control parameters for track continuity and deflection from idealized studies are calculated for Bilis and Toraji. The results are consistent with the theory proposed in Lin et al. For tropical cyclones (TCs) approaching Taiwan from the southeast, the conceptual model proposed by Lin et al. for continuous and discontinuous tracks was applied. For continuous tracks over the CMR, the blocking effect on the outer circulation of the vortex is weak and the vorticity advection around the northern tip is strong due to an intense TC. Weak TCs tend to be totally blocked by the CMR.